Michael Zangl

This plugin allows you to add a watermark images on all images that were uploaded in your wordpress. The watermark is generated on-the-fly and cached for faster access.
To see the plugin in action, select “Add watermark to all images”, chose a watermark image and and submit the settings page. You should then see the watermark appearing on your site. You might need to reload the page (F5 in most browsers) to bypass the browser cache.
The plugin does not change your source images. To disable all watermarks, simply disable the plugin, and everything is back to normal.
If you encounter errors (especially syntax errors), check your PHP version. It should be at least 7.0. This plugin requires Apache and mod_rewrite.

Add Watermarks

Adds watermarks to selected images without changing the original image.

MyWebsiteAdvisor

ChrisHurst

This plugin allows you to Automatically add a watermark to all images as they are uploaded to the WordPress Media Library. 
The plugin uses PNG watermark images with transparency for precise control over the appearance of the watermarks. 
This plugin also supports simple text watermarks with adjustable color, size and transparency. 
The user friendly settings page allows for control over the appearance of your watermark. 
The watermark preview feature allows for easy testing of the plugin settings. 
The watermark size is controlled as a percentage of the target image, 50% means the watermark will be half the width of the target image. 
Watermarks are now removable with the new backup system, any images watermarked while the new backup system is enabled are able to be restored to the original image.
<a href="http://MyWebsiteAdvisor.com/plugins/transparent-image-watermark/" rel="nofollow ugc">Upgrade to Transparent Watermark Ultra</a> for advanced 
watermark features including:
<ul>
<li>Manually Apply Watermarks to Images Previously Uploaded</li>
<li>Fully Adjustable Image Watermark Position</li>
<li>Fully Adjustable Text Watermark Position</li>
<li>Adjustable JPEG Image Output Quality</li>
<li>Highest Quality Watermarks using Image Re-sampling rather than Re-sizing</li>
<li>Lifetime Priority Support and Update License</li>
</ul>
Check out the <a href="http://www.youtube.com/watch?v=fEhZK1U8W94" rel="nofollow ugc">Transparent Image Watermark for WordPress video</a>:
<iframe loading="lazy" class="youtube-player" width="750" height="422" src="https://www.youtube.com/embed/fEhZK1U8W94?version=3&rel=1&showsearch=0&showinfo=1&iv_load_policy=1&fs=1&hl=en-US&autohide=2&hd=1&wmode=transparent" allowfullscreen="true" style="border:0;" sandbox="allow-scripts allow-same-origin allow-popups allow-presentation allow-popups-to-escape-sandbox"></iframe>
Developer Website: http://MyWebsiteAdvisor.com/
Plugin Support: http://MyWebsiteAdvisor.com/support/
Plugin Page: http://MyWebsiteAdvisor.com/plugins/transparent-image-watermark/
Compare Watermark Plugins: http://MyWebsiteAdvisor.com/tools/wordpress-plugins/watermark-plugins-for-wordpress/
Video Tutorial: http://mywebsiteadvisor.com/learning/video-tutorials/transparent-image-watermark-tutorial/
Requirements:
<ul>
<li>PHP v5.0+</li>
<li>WordPress v3.3+</li>
<li>GD extension for PHP</li>
<li>FreeType extension for PHP</li>
</ul>
To-do:

Transparent Image Watermark

Automatically watermark images as they are uploaded to the WordPress Media Library.

Add Watermarks vs Transparent Image Watermark